Considerable quantities of relatively high molecular weight, crosslinked, secondary products are formed in the production of isocyanates on an industrial scale. These secondary products are obtained as non-distillable residues during the working up by distillation of the crude isocyanate solutions obtained in the phosgenation of amines. These residues are generally unsuitable for the conventional application of isocyanates (production of polyurethane plastics). In order to avoid a total loss of the non-distillable residue, it is possible in some cases (for example, in the phosgenation of diamines of the diphenylmethane series) to isolate only part of the pure monomeric isocyanate from the crude phosgenation product, i.e. to leave a considerable proportion of monomeric isocyanate in the sump phase, in order to obtain the relatively high molecular weight secondary products in solution. For the liquid polyisocyanate mixture enriched with relatively high molecular weight products obtained in this way (so-called "crude MDI"), it has been possible in recent years to find a number of potential applications in special plastics.
All attempts, however, to use tolylene diisocyanate enriched with relatively high molecular weight secondary products ("crude TDI") and having a low but still economically acceptable content of monomeric tolylene diisocyanate have so far failed. During distillation of the phosgenation product of tolylene diamines, relatively high molecular weight insoluble products containing uretdione, isocyanurate, carbodimide, uretone imine, urea and biuret groups are formed under the conditions used in practice. Depending on the o-tolylene diamine content of the starting product, methyl benzimidazolones can also be formed during phosgenation and, with time, are biuretized with the free isocyanate groups present, accompanied by formation of insoluble, crosslinked products. Although crude TDI distillation residues which still have a high monomer content (above 80% by weight) and contain virtually no methyl benzimidazolone or its derivatives, are soluble, they are not sufficiently stable in storage. The content of free NCO-groups decreases during storage, even at room temperature, accompanied by an increase in viscosity.
Numerous processes have also been proposed (U.S. Pat. No. 3,634,361, German Offenlegungsschrift No. 2,123,183, German Offenlegungsschrift No. 2,333,150, U.S. Pat. No. 3,455,836 and German Offenlegungsschrift No. 2,423,594) for dissolving TDI distillation residues, which still have a considerable content of free NCO-groups (preferably above 20% by weight), in an organic solvent in the presence of monomeric diisocyanates, optionally at high temperatures, and for using these residue solutions in the conventional way for the polyisocyanate polyaddition process. In practice, however, this method of utilizing the TDI residues fails because of the inadequate stability during storage and the inability to standardize (if, in fact, solutions were possible at all) or because of the sedimentation of insoluble constituents.
In recent years, TDI distillation residues have been partly utilized by means of alkaline hydrolysis. Unfortunately, only relatively small proportions of tolylene diamine can be recovered.
More success in maximizing the TDI yield achieved by using the so-called long-tube vertical evaporators (U.S. Pat. No. 3,897,314) which have been adopted for use on a wide scale in the commercial production of tolylene diisocyanate, and by continuous thermolysis of the liquid TDI residue tar. In the above process, the residues are substantially free from monomers, but still contain free isocyanate groups. This residue has to be stirred into water (quenching) in the form of hot (approximately) 150.degree. to 300.degree. C.) tar-like mass immediately after separation of the pure monomeric TDI in order to avoid smoldering fires and for physiological reasons. During this quenching process, the free isocyanate groups still present react with the water to form additional polyurea groups (hereinafter called denaturing). This reaction is being accompanied by the evolution of carbon dioxide. Only a very small percentage, around 1% to 10% by weight, of included isocyanate groups are left unreacted. Storage in water or in moist form causes the isocyanate content to undergo a further gradual reduction over a prolonged period.
These slag-like TDI residues contain inter alia polyurea, isocyanurate and polycarbodiimide groups and are substantially insoluble in all the conventional solvents. It is only at temperatures above about 250.degree..+-.30.degree. C. that they begin to melt to some extent, decomposing and giving off gasses.
The vast majority of TDI distillation residues are either dumped or burned in furnaces with considerable difficulty. These TDI residue slags amount to approximately 10% of the total TDI production. In cases where TDI residue slags are burned, deposits of firmly adhering, substantially non-flammable tarry masses accumulate on the bottom of the combustion chamber and, in many cases, decompose explosively at temperatures above about 500.degree. C.